1. Field of the Invention
The present invention relates to a light-emitting element, a display device, a light-emitting device, an electronic appliance, and a lighting device each of which uses an organic compound as a light-emitting substance.
2. Description of the Related Art
In recent years, research and development of a light-emitting element (organic EL element) which uses an organic compound and utilizes electroluminescence (EL) have been actively promoted. In the basic structure of such a light-emitting element, an organic compound layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Such a light-emitting element is a self-luminous element and has advantages over a liquid crystal display in having high pixel visibility and eliminating the need for backlights, for example; thus, such a light-emitting element is thought to be suitable for a flat panel display element. A display including such a light-emitting element is also highly advantageous in that it can be thin and lightweight. Besides, very high speed response is one of the features of such an element.
In such a light-emitting element, light-emitting layers can be successively formed two-dimensionally, so that planar light emission can be obtained. Thus, a large-area element can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source which can be applied to a lighting device and the like.
It is important to obtain white light so that the light-emitting element is used for lighting. In general, white light can be obtained with the use of a multicolor light-emitting element which emits light obtained by combining light from a plurality of emission center substances having different emission spectra.
Disclosed in Patent Document 1 is a structure in which a layer for color adjustment is additionally provided in a light-emitting element in which a plurality of light-emitting layers is stacked. However, this structure increases the number of constituent elements, and thus is disadvantageous in terms of cost.